Production of aluminum

ABSTRACT

Production of aluminum by continuous electrolysis of aluminum chloride in molten electrolyte of controlled oxide content.

United States Patent 1191 1111 3,725,222 Russell et al. 1451 Apr. 3,1973 541 PRODUCTION OF ALUMINUM 1,942,522 1/1934 Weber et al ..204/672,919,234 12/1959 Slatin l ..204/67 Inventors: Allen Russell, NewKensmgmn; 3,103,472 9/1963 Slatin ..204/67 Lester PP, Maryville; Warren3,518,172 6/1970 Layne etal ..204/67 E. Haupin, New Kensington, all ofPrimary Examiner-John H. Mack [73] Assignee: Aluminum Company ofAmerica, Assistant Examiner D' valemme Pittsburgh, Attorney-Edward B. Foote [22] Filed: Oct. 26, 1971 57 ABSTRACT 1 1 pp N03 1921653 Productionof aluminum by continuous electrolysis of aluminum chloride in moltenelectrolyte of controlled 52 us. 01 ..204/67 OXIde [Sl] Int. Cl. ..C22d3/12, [58] Field of Search ..204/67 [56] References Cited 17 Claims, NoDrawings UNITED STATES PATENTS 12/1916 McAfee ..204/67 PRODUCTION OFALUMINUM This invention relates to the production of aluminum byelectrolysis of aluminum chloride dissolved in molten salt.

For over 80 years the customary method of producing aluminumcommercially has been the well-known t Hall-Heroult process in whichalumina dissolved in a fluoride bath (principally cryolite) is reducedelectrolytically. Production of aluminum by electrolysis of aluminumchloride dissolved in a molten electrolyte composed of one or morehalides having higher electrodecomposition potential than aluminumchloride (e.g., alkali metal halide or alkaline earth metal halide) hasbeen described in the literature; for example, Z. fur Elektrochemie,Vol. 54, pp. 210-215, U.S. Pat. Nos. 1,296,575, 1,854,684, 2,919,234,3,103,472 and Canadian Pat. No. 502,977.

Although production of aluminum by electrolysis of aluminum chlorideoffers certain potential advantages over the Hall-Heroult process, suchas operation at lower temperature and avoidance of consumption of carbonelectrodes through oxidation by oxygen evolved in electrolysis ofalumina, disadvantages have outweighed such advantages and production ofaluminum by electrolysis of aluminum chloride has not been commerciallyadopted.

Major problems which have effectively precluded commercially economicalcontinuous electrolysis of aluminum chloride dissolved in molten saltsat above the melting point of aluminum stem from the presence of metaloxides such as alumina, silica, titania, and the like in theelectrolytic bath. Metal oxides in the bath, and particularlyundissolved metal oxides, are a primary factor in causing a gradualaccumulation on cell cathodes of a viscous layer of finely dividedsolids, liquid components of the bath, and droplets of molten aluminum.The above-mentioned layer herein referred to as sludge inhibits orinterferes with continued access of electrolytic bath to the cathodes,with the result that upon depletion of the aluminum chloride in thesludge layer by electrolysis, bath solvent in the sludge electrolyzes,with attendantloss of current efficiency in the production of aluminum.The sludge layer also interferes with circulation of the electrolyticbath, with further resultant impairment of electrical efficiency.Moreover, when the bath contains alkali metal halide or alkaline earthmetal halide as the solvent for the aluminum chloride, carbonaceouscathodes of the cell are attacked by alkali metal or alkaline earthmetal produced by electrolysis of such salts, causing spalling anddisintegration of the cathodes, with attendant change in theanode-cathode distance and increase in maintenance expense, as well asintroducing into the electrolyte particles of carbon which contribute tothe formation of sludge at the cathode.

The problem of sludge formation caused by metal oxides in the bath iscomplicated by the fact that, in general, the lower the aluminumchloride concentration in the bath, the better is the electricalconductivity.

From the standpoint of optimum electrical conductivity of the bath, andattendant minimizing of power consumed, it is desirable to operate theelectrolysis process at an aluminum chloride concentration in the bathof about 1 to percent by weight, whereas at such concentrations ofaluminum chloride metal oxides are only very slightly soluble in thebath. Thus, operating in the cell.

desirable aluminum chloride concentration range enhances the problem ofsludge formation.

A further disadvantage of metal oxides in the electrolytic bath is thatdissolved metal oxides have a lower electrodecomposition potential thanaluminum chloride, and upon electrolyzing release oxygen at the cellanodes. Carbon is the most practical material to use for anodes, butevolved oxygen. reacts with the carbon to form gaseous oxides. Suchconsumption of anode carbon affects the operating characteristics of thecell deleteriously by changing the anode-cathode distance, as well asadding to anode expense.

It is the primary object of this invention to improve the production ofaluminum by electrolysis of aluminum chloride, and particularly toincrease the electrical efficiency of the electrolytic cells andotherwise reduce the cost of operation.

In the production of aluminum in accordance with this invention,aluminum chloride dissolved in molten salt of higherelectrodecomposition potential than aluminum chloride such as alkalimetal halide or alkaline earth metal halide is electrodecomposedcontinuously, the concentration of aluminum chloride in the electrolyticbath of salt and aluminum chloride being in the range of l to 15 percentby weight, and preferably 3 to 10 percent by weight, and beingmaintained in such ranges by adding aluminum chloride continuously orintermittently to the bath to replace aluminum chlorideelectrodecomposed. Molten aluminum produced settles out of the bathandcan be withdrawn in any suitable way, such as by tapping or siphoning itfrom the It has been found that in carrying out the above processcontinuously (i.e., for periods of over 700 hours of operation) underconditions in which metal oxides are introduced into the electrolyticbath it is highly important that the concentration of metal oxides inthe bath, expressed as oxygen, be kept below 0.25 percent by weight, andpreferably below 0.1 percent by weight, and more preferably below 0.05percent. Metal oxides, such as alumina, silica, (silicon is consideredherein as a metal, albeit it is a metalloid), iron oxide, titaniumoxide, and lime are only slightly soluble in the electrolytic bath, andas mentioned previously, metal oxides are a primary cause of theformationof the above-mentioned undesirable sludge. Moreover, althoughmetal oxides are only slightly soluble in the bath, electrolysis of thedissolved oxides releases oxygen at carbonaceous anodes of the cell,oxidizing the carbon with resultant increase in the anode-cathodedistance in the cell and an attendant gradual increase in electricalresistance.

By maintaining a concentration of metal oxides in the bath below 0.25percent by weight as aforesaid, the electrolysis of aluminum chloridecan be carried out indefinitely without formation of sludge at thecathodes in amounts which significantly affect the process or equipmentdetrimentally. For example, the processcan be continued indefinitelywith an anode-cathode distance of less than 1 inch, a cathodecurrentdensity of about 10 amperes, a voltage of less than 5 voltsbetween anode and cathode, and a current efficiency of better thanpercent with respect to electrodecomposition of aluminum chloride.

Metal oxides may enter the bath in various ways; for example, asimpurities in bath components (i.e., aluminum chloride or solvent) fedto the cell. Also, moisture which leaks into the cell or is present incell walls or bath components used in the process reacts with moltenaluminum in the cell to form alumina. Likewise, contact of the bath withcell linings or other structural parts of the cell which contain metaloxides, such as refractories containing alumina or silica, can introducesuch oxides into the bath.

In producing aluminum in accordance with this invention, introduction ofmetal oxides into the bath is controlled, as indicated above. Moreover,in continuous operation of the electrolytic process, with attendantcontinuous or intermittent feeding of aluminum chloride to the bath toreplace aluminum chloride which is electrolyzed, it is especiallyimportant in the interest of maintaining the above-mentioned lowconcentration of metal oxides in the bath that the aluminum chloride fedto the bath contain a total of less than 0.25 percent by weight of metaloxides, preferably less than 0.1 percent, and even more preferably lessthan 0.05 percent by weight.

References herein to metal oxides include oxygenated compoundscontaining additional ions besides metal and oxygen, e.g., oxyhalidesand oxynitrides.

In the preferred operation of the process the electrolyte employedconsists essentially of one or more alkali metal halides or alkalineearth metal halides which have a higher electrodecomposition potentialthan aluminum chloride, the chlorides being preferred, and the processis carried out at a temperature below 730 C. but above the melting pointof aluminum (660 C.). For example, a mixture of equal parts by weight ofsodium chloride and lithium chloride is particularly satisfactory as theele ctrolytejlt will be understood that other components can also beadded to the bath, if desired, to modify bath characteristics.

Electrolytic cells of known types employing spaced monopolar electrodes,or spaced bipolar electrodes between anode and cathode terminals, can beused in producing aluminum in accordance with the invention. Aparticularly suitable type of cell is described in U.S. Pat. applicationSer. No. 178,650, of Dell, I-Iaupin and Russell, filed Sept. 8, 1971. Inthe interests of excluding moisture from the cell and recovering gaseousmaterials evolved in the electrolysis it is desirable that the cell beclosed except for one or more outlets for such gaseous materials, andone or more inlets for feeding aluminum chloride into the cell. Asalready indicated, it is also desirable to avoid using in structuralparts of the cell which come in contact with the electrolytic bath anymaterials which introduces metal oxides into the bath. Also, prior tostarting aluminum production in a new'cell it is advisable to removemoisture which is present in the cell walls, electrodes, and otherstructural parts, to avoid reaction of such moisture with moltenaluminum to form alumina in the bath. Such moisture can be driven offconveniently by feeding molten electrolyte into the cell and maintainingit-at elevated temperature long enough that moisture in the cellvaporizes and escapes through openings in the cell, such as outlets forvapors evolved during electrolysis.

The use of the conditions described above make possible substantialeconomics in the production of aluminum by electrolysis of aluminumchloride as a result of improvement in electrical efficiency through 5avoidance of sludge formation and deterioration of the electrodes, andthrough reduced maintenance expense for carbonaceous anodes andcathodes.

If on occasion during operation of the process the amount of undissolvedmetal oxides in the bath happens to increase sufflciently that theconcentration of metal oxides in the bath exceeds a predetermineddesired level, that situation can be corrected by various procedures.Undissolved metal oxide can be separated from the bath, as by filtrationof undissolved oxides from the bath, to reduce the concentration ofmetal oxide in the bath to the desired level. Another alternative is toalter the conditions of operation of the electrolysis processtemporarily so that undissolved metal oxide in the bath is dissolved andelectrolyzed until the concentration of metal oxide in the bath returnsto the desired level. For example, undissolved oxide in the bath can bedissolved by temporarily increasing the capacity of the bath to dissolvemetal oxide present by increasing the concentration of aluminum chloridein the bath, or decreasing the temperature of the bath sufficiently thatenough additional metal oxide dissolves and electrolyzes for theconcentration of metal oxide into the bath to return to thepredetermined desired level, whereupon the prior concentration ofaluminum chloride in the bath, or the prior bath temperature, can berestored.

Another way of increasing the capacity of the bath to dissolve metaloxides so that metal oxide can be removed by electrodecompositionthereof is to add to the bath a component suitable for that purpose. Forexample, when the baths solvent for aluminum chloride is alkali metalchloride, a small concentration of a fluoride e.g., about 1 percent byweight, expressed as fluorine can be introduced into the bath for thatpurpose; magnesium fluoride, aluminum fluoride, sodium fluoride, calciumfluoride, or cryolite are examples of fluorides that can be used.

A further alternative procedure is to reduce the current densityemployed in the electrolytic cell to a level at which the rate ofelectrolysis of dissolved metal oxide increases relative to the rate ofelectrolysis of aluminum chloride, and maintaining such reduced currentdensity until sufficient metal oxide has been electrolyzed that theamount of metal oxide in the bath returns to the desired level.Thereafter the current density can be increased to return it to itsoriginal level.

As a specific example of the invention, aluminum was produced bycontinuous electrolysis of aluminum chloride at 695700 C. in anelectrolytic cell of the type described in the aforesaid U.S. Pat.application of Dell, Haupin and Russell consisting ofa metal shellhaving an electrolysis chamber lined with silicon nitride-bonded fusedsilica, and having a graphite anode in the upper portion thereof and agraphite between each of the opposed electrodes was about 1 inch. Thecell was closed except for an inlet through the top for feeding aluminumchloride into the electrolytic bath, an outlet in the top for chlorineand aluminum chloride vapors generated, and an outlet for withdrawal ofmolten aluminum produced.

The above-mentioned electrolysis compartments were kept immersed inelectrolytic bath consisting essentially of sodium chloride and lithiumchloride, plus about 6-7 percent by weight of aluminum chloride.Aluminum chloride having a total metal oxide content of less than 0.03percent by weight, and essentially free of moisture, was fed into thebath continuously to replace aluminum chloride decomposed and maintainthe aluminum chloride content of the bath at about 6-7 percent byweight.

The cell was operated continuously for 120 days at about 3.3 volts perelectrolysis compartment and an average cathode current density of 8.5amperes per square inch, without noticeable formation of sludge in thebath. The concentration of metal oxides (expressed as oxygen) in theelectrolytic bath remained at less than 0.002 percent by weight of thebath throughout the operation. Molten aluminum produced collected in thelower part of the electrolytic chamber and was drawn off periodically. 5and 2/10 kilowatt hours of electric power was consumed per pound ofaluminum produced.

We claim:

1. In a process for continuous production of aluminum by electrolysis ofaluminum chloride dissolved in molten solvent having a higherelectrodecomposition potential than aluminum chloride, in whichintroduction of metal oxide into the electrolytic bath of aluminumchloride and solvent occurs, and aluminum chloride is fed into the saidbath to replace the aluminum chloride decomposed, the improvementcomprising limiting introduction of metal oxide into the said bathsufficiently that the percent by weight of oxide (expressed as oxygen)in the said bath does not exceed 0.25 percent.

2. A process in accordance with claim 1 wherein the percent by weight ofoxide in the bath does not exceed 0.1 percent.

3. A process in accordance with claim 1 wherein the percent by weight ofoxide in the bath does not exceed 0.05 percent.

4. A process in accordance with claim 1 wherein the said aluminumchloride fed into the electrolytic bath contains less than 0.25 percentby weight of metal oxide.

5. A process in accordance with claim 4 wherein the said aluminumchloride fed into the electrolytic bath contains less than 0.1 percentby weight of metal oxide.

6. A process in accordance with claim 4 wherein the said aluminumchloride fed into the electrolytic bath contains less than 0.05 percentby weight of metal oxide.

7. A process in accordance with claim 1 wherein the concentration ofaluminum chloride in the said bath is maintained at l-l 5 percent byweight.

8. A process in accordance with claim 7 wherein the aluminum chloridefed into the electrolytic bath contains less than 0.25 percent by weightof metal oxide.

9. A process in accordance with claim I wherein upon increase in theconcentration of metal oxide in the bath above a predetermined desiredlevel through introduction of undissolved metal oxide into the bath,sufficient metal oxide is removed from the bath to reduce theconcentration of metal oxide in the bath below the said predeterminedlevel.

10. A process in accordance with claim 9 wherein the removal of metaloxide from the bath is effected by increasing the capacity of the baththru effected by increasing the capacity of the to dissolve metal oxideand thereafter electrolyzing sufficient dissolved metal oxide from thebath that the concentration of metal oxide in the bath returns to. thesaid predetermined desired level.

11. A process in accordance with claim 10 wherein the said increasing ofthe capacity of the bath to dissolve metal oxide is effected byincreasing the concentration of aluminum chloride in the bath. I

12. A process in accordance with claim 10 wherein the said increasing ofthe capacity of the bath to dissolve metal oxide is effected bydecreasing the temperature of the bath.

13. A process in accordance with claim 10 wherein the said solventconsists essentially of alkali metal chloride, and the said increasingof the capacity of the bath to dissolve metal oxide is effected byadding a fluoride to the bath.

14. A process in accordance with claim 9 wherein the said removal ofmetal oxide from the bath is effected by temporarily decreasing thecurrent density sufficiently to increase the rate of electrolysis ofmetal oxide from the solution.

15. A process in accordance with claim 9 wherein the said removal ofmetal oxide is effected by filtration of undissolved oxides from thebath.

16. A process in accordance with claim 1 wherein the said solventconsists essentially of at least one salt selected from the groupconsisting of alkali metal halides and alkaline earth metal halides.

17. A process in accordance with claim 1 wherein the said solventconsists essentially of at least one salt selected from the groupconsisting of alkali metal chlorides and alkaline earth metal chlorides.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,725,222 Dated April 3, 1973 Inventor(s) It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Col. 4, line 2 Change "economics" to "economies".

Claim 10, lines3 and 4 After "bath" delete "thru effected by increasingthe capacity of the Signed and seaiLed this 20th day of November 1973.

(SEAL) Attest:

EDWARD, M.FLETCHER,JR. RENE D. TEGTME YER v Attesting Officer I ActingCommissioner of Patents FORM PO-lOSO (10-69) USCOMM-DC 60376-P69 a 11.5,GOVERNMENT PRINTING OFFICE: (969 o-3ss-:3A

2. A process in accordance with claim 1 wherein the percent by weight ofoxide in the bath does not exceed 0.1 percent.
 3. A process inaccordance with claim 1 wherein the percent by weight of oxide in thebath does not exceed 0.05 percent.
 4. A process in accordance with claim1 wherein the said aluminum chloride fed into the electrolytic bathcontains less than 0.25 percent by weight of metal oxide.
 5. A processin accordance with claim 4 wherein the said aluminum chloride fed intothe electrolytic bath contains less than 0.1 percent by weight of metaloxide.
 6. A process in accordance with claim 4 wherein the said aluminumchloride fed into the electrolytic bath contains less than 0.05 percentby weight of metal oxide.
 7. A process in accordance with claim 1wherein the concentration of aluminum chloride in the said bath ismaintained at 1-15 percent by weight.
 8. A process in accordance withclaim 7 wherein the aluminum chloride fed into the electrolytic bathcontains less than 0.25 percent by weight of metal oxide.
 9. A processin accordance with claim 1 wherein upon increase in the concentration ofmetal oxide in the bath above a predetermined desired level throughintroduction of undissolved metal oxide into the bath, sufficient metaloxide is removed from the bath to reduce the concentration of metaloxide in the bath below the said predetermined level.
 10. A process inaccordance with claim 9 wherein the removal of metal oxide from the bathis effected by increasing the capacity of the bath thru effected byincreasing the capacity of the to dissolve metal oxide and thereafterelectrolyzing sufficient dissolved metal oxide from the bath that theconcentration of metal oxide in the bath returns to the saidpredetermined desired level.
 11. A process in accordance with claim 10wherein the said increasing of the capacity of the bath to dissolvemetal oxide is effected by increasing the concentration of aluminumchloride in the bath.
 12. A process in accordance with claim 10 whereinthe said increasing of the capacity of the bath to dissolve metal oxideis effected by decreasing the temperature of the bath.
 13. A process inaccordance with claim 10 wherein the said solvent consists essentiallyof alkali metal chloride, and the said increasing of the capacity of thebath to dissolve metal oxide is effected by adding a fluoride to thebath.
 14. A process in accordance with claim 9 wherein the said removalof metal oxide from the bath is effected by temporarily decreasing thecurrent density sufficiently to increase the rate of electrolysis ofmetal oxide from the solution.
 15. A process in accordance with claim 9wherein the said removal of metal oxide is effected by filtration ofundissolved oxides from the bath.
 16. A process in accordance with claim1 wherein the said solvent consists essentially of at least one saltselected from the group consisting of alkali metal halides and alkalineearth metal halides.
 17. A process in accordance with claim 1 whereinthe said solvent consists essentially of at least one salt selected fromthe group consisting of alkali metal chlorides and alkaline earth metalchlorides.